Optical storage devices such as optical disks and optical tape are becoming more and more extensively used. It is, of course, highly desirable to increase the storage capacity and other performance parameters such as signal to noise and storage density.
A multilayer optical storage device is set forth in SPIE Vol. 2338, p. 247, 1994 (K. A. Rubin, H. J. Rosen, W. W. Wang, W. Imaino, and T. C. Strand). The recording layers consist of two or more recording surfaces spaced sufficiently far apart that each surface can be recorded and read independently. A two layer medium might be expected to offer twice the density of a conventional single layer medium. Generally, this is not true because the return light beam is severely attenuated relative to the return light beam found in a conventional single layer medium. For example, an attempt to read a layer that is not the layer closest to the radiation source requires the light to pass through the other layers with a consequent loss of intensity. Even reading the layer closest to the radiation source does not generate the maximum possible return beam because this layer must be optimized for transparency, not merely optimal return beam. It might, for example, be very thin and unreflective compared to a conventional single layer.
This severe attenuation of the return light beam substantially reduces the carrier. More significantly, the carder to noise ratio (CNR) also decreases because, unlike the carrier, the noise power usually consists of large contributions from sources not directly proportional to the beam intensity and thus not proportionately reduced by the attenuation. This reduction in CNR reduces the storage density of the layer and the overall storage device.